Simple reallythe right shoulder cant move unless the left one does. theright cross is generated of the back foot plus the left stabalises the right shoulder and gives it direction while stabalising the weight on the back foot to generate the power plus giving head the stability it reqiures through the correct posture. Stopping you leaning into whatevers coming in your direction, and it wont hurt as much if it lands. Also your eyes see everthing so you have a better chance of not getting hit which is what its all about, got it